1. Field of the Invention
This invention relates generally to nucleic acid sequences encoding proteins that are associated with abiotic stress responses and abiotic stress tolerance in plants. In particular, this invention relates to nucleic acid sequences encoding proteins that confer drought, cold, and/or salt tolerance to plants.
2. Background Art
Abiotic environmental stresses, such as drought stress, salinity stress, heat stress, and cold stress, are major limiting factors of plant growth and productivity. Crop losses and crop yield losses of major crops such as rice, maize (corn) and wheat caused by these stresses represent a significant economic and political factor and contribute to food shortages in many underdeveloped countries.
Plants are typically exposed during their life cycle to conditions of reduced environmental water content. Most plants have evolved strategies to protect themselves against these conditions of desiccation. However, if the severity and duration of the drought conditions are too great, the effects on plant development, growth and yield of most crop plants are profound. Furthermore, most of the crop plants are very susceptible to higher salt concentrations in the soil. Continuous exposure to drought and high salt causes major alterations in the plant metabolism. These great changes in metabolism ultimately lead to cell death and consequently yield losses.
Developing stress-tolerant plants is a strategy that has the potential to solve or mediate at least some of these problems. However, traditional plant breeding strategies to develop new lines of plants that exhibit resistance (tolerance) to these types of stresses are relatively slow and require specific resistant lines for crossing with the desired line. Limited germplasm resources for stress tolerance and incompatibility in crosses between distantly related plant species represent significant problems encountered in conventional breeding. Additionally, the cellular processes leading to drought, cold and salt tolerance in model, drought- and/or salt-tolerant plants are complex in nature and involve multiple mechanisms of cellular adaptation and numerous metabolic pathways. This multi-component nature of stress tolerance has not only made breeding for tolerance largely unsuccessful, but has also limited the ability to genetically engineer stress tolerance plants using biotechnological methods.
Therefore, what is needed is the identification of the genes and proteins involved in these multi-component processes leading to stress tolerance. Elucidating the function of genes expressed in stress tolerant plants will not only advance our understanding of plant adaptation and tolerance to environmental stresses, but also may provide important information for designing new strategies for crop improvement.
Expression and function of abiotic stress-inducible genes have been well studied at a molecular level. Complex mechanisms seem to be involved in gene expression and signal transduction in response to the stress. These include the sensing mechanisms of abiotic stress, modulation of the stress signals to cellular signals, transduction to the nucleus, second messengers involved in the stress signal transduction, transcriptional control of stress-inducible genes and the function and cooperation of stress-inducible genes.
In animal cells, phosphatidylinositol-specific phospholipase C (PI-PLC) plays a key role in early stages of various signal-transduction pathways. Extracellular stimuli such as hormones and growth factors activate PI-PLCs. PI-PLC hydrolyzes phosphatidylinositol 4,5-biphosptate (PIP2) and generates two second messengers, inositol, 4,5-triphosphtate (IP3) and 1,2-diacylglycerol (DG). IP3 induces the release of intracellular Ca2+ into the cytoplasm, which in turn causes various responses therein. DG and PIP2 also function as second messengers and control various cellular responses.
In plants, similar systems are thought to function in abiotic stress response. It is clearly demonstrated that phospholipases A, C or D (PLA, PLC or PLD), depending upon their site of cleavage, play a role in the early signal transduction events that promote the cell volume changes associated with osmotic stress and osmoregulation in plants which is important for plant stress tolerance (Wang X. et at., 2000, Biochemical Society Transactions. 28; 813–816; Chapman KD, 1998 Tre. Plant Sci. 3:419–426). For example, in guard cells, abscisic acid (ABA)-induced stomatal closure is mediated by rapid activation of PIP2-PLC. This leads to an increase in IP3 levels, a rise in cytosolic calcium, and the subsequent inhibition of K+ channels. Recently, a gene for phospholipase C, AtPLC was demonstrated to be rapidly induced by drought and salt stresses in Arabidopsis thaliana (Hirayama, T. et al., 1995 Proc. Natl. Acad. Sci. 92:3903–3907).
As mentioned above, Ca2+ ions play important roles as second messengers in various signal-transduction pathways in plants. Marked increase in intracellular Ca2+ concentration has been observed upon stimulation by wind, touch, abiotic stresses (cold, drought and salinity) or fungal elicitors. Several genes for Ca2+ binding proteins with a conserved EF-hand domain have been isolated and showed increased expression level upon abiotic stress treatment (Frandsen G. et al., 1996 J Biol. Chem. 271:343–348; Takahashi S. et al., 2000 Pant Cell Physiol. 41:898–903).
The enigmatically named 14-3-3 proteins have been also the subject of considerable attention in recent years since they have been implicated in the regulation of diverse physiological processes in eukaryotes ranging from slime moulds to higher plants. In plants, many biological roles for 14-3-3 proteins have been suggested. The most significant of these include roles in the import of nuclear encoded chloroplast proteins, in the assembly of transcription factor complexes and in the regulation of enzyme activity in response to intracellular signal transduction cascades (Chung H J. et al., 1999 Tre. Plant Sci. 4:367–371). The native 14-3-3 proteins are homo- or heterodimers and, as each monomer has a binding site, a dimer can potentially bind two targets, promoting their association. Alternatively, target proteins may have more than one 14-3-3-binding site.
Several functions have been proposed for the 14-3-3 proteins in terms of involvement of plant stress tolerance. The 14-3-3 proteins could function as regulators in stress signal transduction. For example, RCI14A and RCI14B genes are induced by cold treatment in Arabidopsis and are highly homologous to the 14-3-3 proteins. The rise in the RCI transcript levels observed in response to cold treatment suggests a role for the RCI proteins in the stress signaling transduction pathway (Jarillo J A et al., 1994 Plant Mol. Biol. 25:693–704)
Due to the commercial consequences of environmental damage to crops, there is an interest in understanding the stress response signal transduction mechanisms in plants and how these can be manipulated to improve a plant's response to environmental damage. There is a need, therefore, to identify genes expressed in stress tolerant plants that have the capacity to confer stress resistance to its host plant and to other plant species. Newly generated stress tolerant plants will have many advantages, such as increasing the range that crop plants can be cultivated by, for example, decreasing the water requirements of a plant species.